[PATCH v4] arm64: kernel: implement fast refcount checking

[Kees Cook]

On Mon, Jul 31, 2017 at 12:22 PM, Ard Biesheuvel
<ard.biesheuvel at linaro.org> wrote:
> v4: Implement add-from-zero checking using a conditional compare rather than
>     a conditional branch, which I omitted from v3 due to the 10% performance
>     hit: this will result in the new refcount to be written back to memory
>     before invoking the handler, which is more in line with the other checks,
>     and is apparently much easier on the branch predictor, given that there
>     is no performance hit whatsoever.

So refcount_inc() and refcount_add(n, ...) will write 1 and n
respectively, then hit the handler to saturate? That seems entirely
fine to me: checking inc-from-zero is just a protection against a
possible double-free condition. It's still technically a race, but a
narrow race on a rare condition is better than being able to always
win it.

Nice!

-Kees

-- 
Kees Cook
Pixel Security